Holograms and diffraction gratings are images that diffract light created by the texturizing of a substrate under heat and pressure. Such images are used to create decorative packaging, security products and a host of other uses. The embossed substrates are often metallized to create high contrast. Such metallized substrates are found on credit cards, membership materials, board laminates such as packaging materials, labels, toys and many commodity products.
Currently, it is well known in the art to produce holograms by embossing polyvinylchloride (PVC), polyethyleneterephthalate (PET), biaxially oriented polypropylene (BOPP), polystyrene (PS), polyamides (PA) such as Nylon or other plastic materials. If the substrates are BOPP or PET, it is also well known in the art to produce holographic substrates by coating a relatively thick acrylic layer on the substrate by a coating process. In the case of PET substrates, the coating is applied in an off-line process as is often done by hologram manufacturers at the point of use of the web substrate. However, it is desirable to obtain a pre-coated and embossable PET film from a substrate manufacturer that can directly accept the holographic texture. Such a material would obviate the need for the hologram manufacturers to coat the base materials and reduce overall costs of manufacturing.
Unfortunately, in order to produce a directly embossed film at the point of film manufacture, it has been necessary to provide a thick, embossable surface on the PET film. Such a surface can be provided either through a co-extrusion process or, perhaps, through an inline coating process. In the case of co-extrusion, it is necessary to produce a surface layer with many of the same characteristics of PET. Therefore, IV, melt strength and the like are necessary to get the co-extruded layer through the PET film making process. Typical materials that can survive this process are often analogs of PET itself. These materials suffer the problem of having low crystallinity and are, therefore, heat-sealable. A heat-sealable material will often stick to the embossing shim rendering the embossed texture of little commercial quality.
In the case of inline coating of PET film, the thickness deemed necessary to produce embossable holographic substrates is practically impossible to achieve with a film making process. A thick-coated layer will require reduced PET film making line speeds to accommodate the high water loads in a tenter oven. In addition, the presence of a thick coating on the PET film often makes recycling of coated PET film waste back into the process difficult, or in many cases impossible.
U.S. Pat. No. 4,913,858 describes the requirements of an offline coating for holographic embossing use.
U.S. Pat. No. 3,758,649 describes embossing directly into a thermoplastic sheet.
Other known publications include:
xe2x80x9cCreating Interference Colors on Thermoplastic Films Without Colorantsxe2x80x9d, Trudy Bryson, Coburn Corporation, 1982
xe2x80x9cDimension, design and printabilityxe2x80x9d, James Coburn
xe2x80x9cHolographic Advances Open New Dimensions for Convertersxe2x80x9d, S. F. Mann, Dennison Mfg. Co., 1986
xe2x80x9cOptical Embossingxe2x80x9d, James River Products
This invention relates to a composite polyethyleneterephtlate film adapted as a directly embossable substrate for holographic use, wherein the film is produced by inline coating uniaxially oriented PET film, drying and then transverse stretching the coating and film to produce the composite film.
The invention also relates to a directly embossable, coated polyethyleneterephthalate film including a dry, uniaxially oriented PET film, and a coating applied to the PET film, wherein the coating and the PET film have as a composite been transversely stretched, the film being coated from an aqueous solution with a non-crosslinked polystyrene-acrylic emulsion or a non-crosslinked polyester dispersion or a mixture thereof, the Tg of the coating resin being greater than about 20xc2x0 C., preferably greater than about 35xc2x0 C., and less than about 70xc2x0 C., the coating resin being capable of impregnating a surface of the PET film on drawing, rendering the film surface susceptible to embossing under pressure and the coating having low heat sealability.
The invention further relates to a method of producing a coated, directly embossable polyethyleneterephthalate film including stretching a polyterephthalate film to form a uniaxially-oriented polyethyleneterephthalate film, drying the uniaxially oriented polyethyleneterephthalate film, coating at least one surface of the uniaxially oriented polyethyleneterephthalate film with an aqueous solution of an organic material, drying the coating to form a coated uniaxially oriented polyethyleneterephthalate film, rendering at least one surface of the coated uniaxially oriented polyethyleneterephthalate film susceptible to direct embossing by impregnation of the surface of the uniaxially oriented polyethyleneterephthalate film with at least a portion of the coating by transverse stretching the coated, uniaxially oriented polyethyleneterephthalate film.
In yet another aspect, the invention relates to a method of producing a directly embossable substrate having low heat sealability comprising inline coating a uniaxially oriented PET film with about 0.1 xcexcm to about 0.2 xcexcm in thickness of a non-crosslinked polystyrene-acrylic emulsion or non-crosslinked polyester dispersion, drying the coating, and transverse stretching the resulting coated film.